The present invention relates to a beam-type caliper. More particularly this invention concerns a dial caliper for standard (British or American) and metric readout.
A beam-type caliper generally has an elongated beam on which is displaceable a slider. A pinion carried on the slider is in constant mesh with the teeth of a rack extending longitudinally along the beam. An indicator is connected via an arbor to this pinion and reads out the spacing between a pair of jaws or fingers one of which is carried on the beam and the other of which is carried on the slider.
In one such arrangement the indicator is constituted as an odometer-type counter having an input gear or sprocket which constitutes the pinion that meshes with the rack. The entire counter is pivotal on the slider and a relatively stiff leaf spring is provided to pivotally bias the entire counter in a direction to force the input sprocket thereof into tight mesh with the rack. As the counter is a relatively massive unit it is essential that the spring force be relatively great so that the slider cannot easily slide along the beam. Furthermore the likelihood of the sprocket or pinion damaging the teeth of the rack is greatly increased if the spring force is decreased so as to protect the relatively fine and delicate teeth of the rack. A shock might disengage the pinion from the rack and therefore cause the counter to give an incorrect reading as to the relative positions of the jaws of the caliper. It is therefore necessary in such an arrangement carefully to construct the spring so that the spring force is not so great as to damage the teeth but not so light as to allow the pinions to come out of engagement with these teeth. One advantage must be traded off against another so that rarely is a satisfactory system obtained.
Another disadvantage of this known system is that it is necessary to replace the entire counter mechanism when the pinion thereof becomes worn. Even though a caliper is extremely carefully machined so that its various parts are of highly expensive construction, it is normally not economically justifiable in such an arrangement to replace the counter. Furthermore the journalling of the shaft or arbor carrying the pinion is often inadequate so that is is subjected to excessive bending forces and therefore frequently wears and works loose.
The above-described difficulties are compounded in a dial-type caliper having more than one readout, as for standard (British or American) and metric units of length. In such an arrangement the ardor for the pinion is typically journalled directly in the housing of the dial. It is therefore necessary very carefully to machine the seat for the arbor of the pinion. Once again biasing means is necessary to urge the pinion into the rack with a force which is great enough to prevent slippage of the pinion relative to the rack but not so great as to damage either of these elements. This, therefore, requires extremely careful and expensive machining. Furthermore once the device is somewhat worn whatever carefully set biasing force was originally present will be lost. In another known arrangement a spring is provided having a forked end each of whose sides is formed with a respective semi-cylindrical cutout. This forked end embraces the arbor of the pinion and biases it against the rack. Once again this arrangement requires extremely careful establishing of the spring force with the concomitant expensive production of the biasing parts. Nonetheless the biasing is effective over an extremely narrow area at only one location on the pinion so that wear is likely and the system therefore has a short service life. Furthermore once the journal is worn it is necessary to exchange almost the entire dial assembly, thereby greatly increasing the repair cost of such a caliper.